1. Field of the Invention
This invention relates to a light activated switch system and more particularly to a light activated switch system using a semiconductor controlled rectifier as a switch.
2. Description of the Prior Art
In the operation of a conventional light activated silicon switch (LASS), the di/dt capability i.e., the rate of current increase or "turn on" as a function of time, which can be delivered to a load depends, inter alia, on the rate of rise of the intensity of light impinging on the LASS. The di/dt capability of a particular LASS is a major restriction on performance and many methods have been proposed to remedy the problem including external circuits for limiting the current rise on switching of the LASS. However, external circuits cause power losses and time lags.
In order to initiate switching anywhere across the conducting area of the switch, the light intensity impinging thereon must exceed some minimum value I.sub.o. This value is determined by the current level provided by optically excited charge carriers. The current level must be sufficient to sustain regenerative injection of minority carriers by the applied voltage if the device is to remain "ON" after illumination terminates. The difficulty is that only a small portion of the device is responsive to the light activation and initially switches to the conducting state. The device is dependent on carrier diffusion to "turn on" the remainder of the active region, which requires substantial time. Meanwhile, on "turn on" the voltage across the device drops instantaneously to about 10% of the blocking state value. Thus, the current is shunted through the portions of filaments of the device in the conducting state, causing a very high current density and fusion of the device.
The di/dt capability, or more accurately the switching time characteristic of the device, can be improved by using light sources having a very fast rise time (short rise period) such as Nd lasers, to impinge on the switch causing efficient switching and preventing a large amount of power from dissipating in the switch and burning it out. However, lasers and many other light sources having fast rise times are expensive to build and to operate.
The present invention provides a light activated switch system capable of fast switching times using conventional light sources and LASS and wherein the intensity and rise time of the light source is non-critical.